1. Field of the Invention
The present invention is in the field of fire alarms and more specifically relates to a fire-detecting system that uses a microprocessor for varying the alarm threshold to adapt the fire detector to varying ambient conditions, so as to increase the speed of response of the system and to reduce its false alarm rate.
2. The Prior Art
The related U.S. patent application Ser. No. 07/851,869 describes the structure and operation of a carbon dioxide sensor that produces an output signal representative of the concentration of carbon dioxide in the air immediately surrounding the sensor. In contrast, the emphasis in the present application is on the use made of the output signal for determining when a fire is present. In U.S. Pat. No. 5,053,754 issued Oct. 1, 1991 to the present inventor for "Simple Fire Detector" and in U.S. Pat. No. 5,103,096 issued Apr. 7, 1992 to the present inventor for "Rapid Fire Detector" there were described fire detectors that employ carbon dioxide sensors. In each of those detectors, a fixed threshold is employed, and an alarm signal is generated when the sensed carbon dioxide level, or its rate of change, exceeds the threshold.
If there were absolutely no carbon dioxide present in the atmosphere, detection of fires would be greatly simplified, because any amount of carbon dioxide detected would indicate the present of a fire. However, in reality there is always a small amount of carbon dioxide in the atmosphere, and its concentration ranges from approximately 400 parts per million outdoors to, typically, 1000 parts per million in an office building when people are present.
Thus, depending on where a fire detector is located, the ambient concentration of carbon dioxide may vary with the situation by a factor of three or more. Further, even at a particular location, such as in an office building, the concentration of carbon dioxide may vary in time so that the maximum concentration may be several times the minimum concentration at that location.
These relatively wide variations in the ambient carbon dioxide concentration must be taken into consideration when a threshold is set. In mass-produced fire sensors such as the widely-used domestic smoke detector, the threshold is set at the factory. Due to the above-described variations in the background level of carbon dioxide, it is necessary to set the threshold at a comparatively high level to avoid the occurrence of excessive false alarms. Unfortunately, setting the threshold high renders the instrument less sensitive thereby resulting in a delayed response. A delayed response is very undesirable because of the importance of detecting a fire as early as possible.
Thus it is seen that a fire detector having a fixed threshold is at a disadvantage. Due to the fluctuations in the concentration of carbon dioxide from one location to another and from time to time at any particular location, the threshold must be set relatively high to keep the false alarm rate acceptable, and if this is done, the instrument is necessarily less able to respond quickly to a fire.